The invention relates to a water-cooled catalyst system for reducing the emission of pollutants in the form of hydrocarbons, carbon monoxides and nitrous oxides from fuelled combustion engines, in particular ship engines or stationary engines
An auto-ignition or a fuelled combustion engine with an external ignition system essentially generates carbon dioxide but also carbon monoxide, nitrous oxides and hydrocarbons which are emitted in the exhaust gases. Whereas the carbon and nitrous oxides are chemically defined compounds, the hydrocarbons comprise a large number of compounds.
Today, increasingly strict exhaust gas standards are being passed which now allow only very small residual quantities of the said toxic gases. This applies not only to car engines but increasingly also to ship engines and stationary engines. In particular, the increasing numbers of motor boats travelling on lakes and rivers will in the near future be subject to stricter regulations which allow scarcely any exhaust emissions from engines presently in use.
Exhaust cleaning technologies of petrol driven engines focus on catalysts controlled by a lambda probe, in particular three-way catalysts, but depending on the country also oxidation and/or double bed catalysts. These catalysts reach a high temperature in the range from 900.degree. to 1000.degree. C. after a short time, i.e. they develop a great heat radiation and must lie out of human reach. In cars this is not a problem, the catalysts are safely arranged below the vehicle floor at a relatively large distance from the engine, and exposed to cooling air flows. A boat engine however only allows catalytic exhaust gas detoxification if the catalyst is fitted in the immediate vicinity of the engine, where there is largely no cooling wind. In view of the heat developed by the catalyst and the associated risk of fire, despite the wide range on the market almost no boat engines can be found with catalytically cleaned exhaust gases. This is all the more disadvantageous as it is naturally more difficult for a boat to escape quickly and completely from its own exhaust gas cloud. In unfavourable wind conditions, such an exhaust gas cloud can accompany the boat for relatively long periods.
EP, B1 0314129 describes a catalyst system for petrol engines, using which exhaust gases from boat engines can be treated even when the injection mixture is rich, i.e. with high CO and HC emissions. This is aimed in particular at boat engines which are operated with very high lambda values of between 0.75 and 0.90. To solve the problem, a catalyst system is used with a reductive part lying upstream in the exhaust gas flow and an oxidative catalyst part lying downstream, with a purely axial guidance of the exhaust gas through the catalyst parts. The catalyst is surrounded by a water-cooled housing, so it cannot develop a great heat radiation. According to a feature essential to the invention, between the catalyst shell and the water-cooled housing is an air gap. Secondary air is drawn from the environment on the downstream end of the catalyst, guided through the said air gap into an intermediate chamber between the reductive and the oxidative catalyst parts and there expelled with the exhaust gases. The secondary air introduced via a special pump serves, in addition to secondary combustion of the rich injected fuel mixture, also to insulate the very hot catalyst from the cold water-cooled housing parts. The risk of fire on any leakage of exhaust gas into the boat in the event of a defect in the secondary air fan or secondary air line is eliminated by non-return valves.
The inventor has faced the task of creating a water-cooled catalyst system of the type described initially which can be used universally for all engine and catalyst types and which requires no additional pipelines with non-return valves.